1. Field of the Invention
The present invention is directed to improving the optical signal transmitted to optical detectors or arrays of optical detectors, and, more particularly, to eliminating spectral variation indicating optical interference generated by the presence of a layer on the active area of an optical detector or arrays of optical detectors.
2. Description of Related Art
The clarity of optical signals transmitted to optical detectors is important, given that the distortion of these signals leads to erroneous detector responses and, consequently, erroneous interpretations of data. However, spectral distortion commonly results from the presence of layers on the active areas of detectors. Such layers may be intentionally-placed optical layers such as passivation layers or may be residual layers (such as oxide layers) accumulated as a result of numerous processing steps. Regardless of the origin or purpose of the layer, if its refractive index differs sufficiently from that of the detector material, multiple reflections at the air-to-layer and layer-to-detector interfaces will cause optical interference. This interference manifests itself as a distortion or "spectral variation" in the optical signal. Hence, the presence of a layer may adversely affect the spectral content of optical signals transmitted to the optical detector.
The problem of spectral variation is compounded when detectors are grouped together in arrays. If a thin optical layer is deposited on the individual detectors, but the thickness of the layer is not adequately controlled for uniformity across the array, the spectral responsivity of the individual detectors will differ across the array. This can significantly complicate the interpretation of the data and/or reduce the utility of the data collected due to the interaction of scene spectra variations with detector-to-detector spectral response variations.
The spectral variation caused by such layers is apparent when the detector responsivity is plotted versus wavelength. However, such spectral variation becomes nearly periodic if the responsivity is plotted versus wavenumber (wavenumber is equal to the reciprocal of the wavelength). More particularly, for a layer having a refractive index n, a thickness t, and with optical input incident at an angle of incidence .theta.' within the layer, the interference portion of the detector response will exhibit a periodicity in wavenumber of period EQU 1/(2nt cos .theta.'),
given constant relative refractive indices. For example, a residual oxide layer with an accumulated thickness of 2.7 .mu.m and a refractive index of 1.5 will result in a 0.12 .mu.m.sup.-1 periodicity, given a normal incidence, i.e., .theta.'=0.degree., of the optical signal. The amplitude of the interference period is dependent on the refractive indices of the incident medium, the layer itself, and the optical detector.
Several approaches have been used to achieve a reduction in undesirable spectral variation stemming from the presence of layers on optical detectors. For example, anti-reflection coatings have been applied to such layers to reduce distortion. However, the additional processing necessary to produce an effective anti-reflection coating may be impractical or even impossible for some types of detectors. Further, an anti-reflection coating may merely reduce, but not eliminate, the spectral variation. Another approach involves making the optical layer as thin as possible such that the period is increased. While this will not eliminate the effect of the interference, it will increase the period of the interference and, for an array of detectors, will result in smaller spectral differences among detectors for a given fractional thickness variation across the array. However, such a reduction of the thickness of an optical layer may compromise other detector properties.
Thus, a need remains for a practical method to eliminate spectral variation generated by the presence of a layer on an optical detector without sacrificing detector properties. The present invention fulfills these needs.